In general, this invention relates to capacitive devices such as capacitors and filters. In particular, this invention relates to such devices constructed of metallic electrodes attached to ceramic dielectrics.
To reduce the physical size of such devices, and retain their electrical capacity, ceramic materials are frequently used as dielectrics instead of air or other dielectric materials. Using a ceramic material for a dielectric in combination with a metallic electrodes permits a substantial reduction in the physical size of a capacitor or filter while retaining or even increasing its electrical capacitance.
Reducing the physical size of a capacitor by using dielectrics is not without drawbacks however. As the physical size of the capacitor decreases, the dimensions of electrodes attached to the dielectric must of course also decrease. As the frequency of a signal applied to the capacitor increases and the physical size of the electrodes decreases, "skin effect" losses in the electrodes become more substantial. Further reductions in physical size of the capacitor must accommodate increasing skin effect losses attributable to smaller electrode dimensions.
One solution to the problem of increased skin effect losses in small capacitive devices operated at high frequencies is the use of superconducting electrodes attached to a ceramic dielectric. The electrical resistance of a superconductor is practically zero when the superconductor is at or below its transition temperature. Since skin-effect losses are related to electrical resistance, such losses are minimized by the use of superconductors.
One recently discovered superconductor, Ytrium-BariumCopper-Oxide, (YBa.sub.2 Cu.sub.3 O.sub.x) or YBC as it is know in the art, is particularly attractive to use as an electrode because it superconducts above the temperature of liquid nitrogen. Because of its relatively high transition temperature, YBC, when applied to a ceramic substrate, can be used to provide a superconducting electrode.
A problem with using YBC as a superconducting electrode with conventional, prior art ceramics has been the relatively large difference between the thermal expansion coefficients of YBC and prior art ceramic dielectrics. When YBC is deposited on a conventional ceramic dielectric substrate and cooled to its transition temperature, mechanical stresses between the YBC and ceramic caused by differences between thermal expansion coefficients create stresses that cause micro-cracks to develop at the interface between the dielectric and superconductor after temperature cycling. The micro-cracks between the ceramic and YBC create a resistance which defeats the purpose of using the superconductor. Repetitive temperature cycling increases the number of micro-cracks which further reduces the effective conducting cross-section and increases the resistance of the superconductor. In addition, the micro-cracks developed between the dielectric ceramic and the superconductor electrode will deteriorate the effective dielectric "Q" or quality factor due to the reduction of coupling between dielectric and electrodes.
Known ceramic materials which have compatible thermal expansion coefficients have dielectric constants which are relatively low, reducing their usefulness as filters or capacitors at high frequencies. Some ceramics having compatible thermal expansion coefficients and high dielectric constants have been found to adversely affect the superconductor, lowering its transition temperature. This effect caused by the ceramic is known as "poisoning" of the superconductor.
It is therefore an object of the present invention to provide an electrical device having a ceramic dielectric with a thermal expansion coefficient compatible with YBC electrodes.